Method for producing a group III nitride semiconductor light-emitting device

BACKGROUND OF THE INVENTION

00011. Field of the Invention

0002The present invention relates to a method for producing a Group III nitride semiconductor light-emitting device, and more particularly to a method for forming a p contact layer.

00032. Background Art

0004In recent years, there has been an expanding demand for general illumination applications of Group III nitride semiconductor light-emitting devices, and high-power light-emitting devices are being developed. Every year, the driving current is increasing with the development of high-power light-emitting devices. Therefore, the power consumption is remarkably increased due to resistance, causing reduction in light emission performance. To avoid this, the resistance of light-emitting devices must be reduced.

0005Most of Group III nitride semiconductor light-emitting devices currently produced are of a lateral conductive face-up type or flip-chip type. In the face-up type device, an ITO transparent electrode is formed on almost the entire top surface of the p contact layer. In the flip-chip type device, a reflecting electrode of high-reflectivity metal such as Ag or its alloy is formed on almost the entire top surface of the p contact layer. To reduce the resistance of light-emitting devices, the contact resistance between the p contact layer and the electrode may be reduced. Improved light emission performance and uniform light emission distribution can be expected by reducing the contact resistance.

0006In a known method to reduce the contact resistance between the p contact layer and the electrode, p-InGaN is used as a p contact layer instead of conventionally known p-GaN. InGaN has a smaller work function than GaN, and has a work function closer to that of the electrode material. Thereby, the contact resistance can be reduced. Japanese Patent Application Laid-Open (kokai) No. 2010-62254 discloses a p-type contact layer comprising a p-type first contact layer formed of p-InGaN and a p-type second contact layer formed of p-GaN in this order from and on a p electrode. Such a structure of the p contact layer can reduce the contact resistance between the p contact layer and the electrode. A method is also well known to restrain evaporation of In and improve crystallinity of InGaN by employing nitrogen as a carrier gas when forming InGaN by MOCVD.

0007To reduce the resistance of the device, p-type activation is required for a Group III nitride semiconductor doped with Mg. This is usually done by thermal treatment in an atmosphere containing no hydrogen.

0008However, this method may degrade crystallinity. A method disclosed by Japanese Patent Application Laid-Open (kokai) No. 2005-159341 is also suggested. Japanese Patent Application Laid-Open (kokai) No. 2005-159341 discloses a method for p-type activation of a Group III nitride semiconductor without degradation of crystallinity, the method comprising: forming a Group III nitride semiconductor doped with a p-type dopant in a hydrogen or ammonia atmosphere at a temperature of 1000° C. or higher, lowering the temperature by 50° C. or more to maintain at 900° C. or higher, and thereafter stopping the supply of hydrogen or ammonia to replace with an inactive gas such as nitrogen.

0009Even with the method disclosed by Japanese Patent Application Laid-Open (kokai) No. 2010-62254 or 2005-159341, however, difficulty is encountered in reducing the contact resistance with the electrode while achieving p-type conduction of the p contact layer formed of InGaN. The resistance of the device cannot be sufficiently reduced. Therefore, both the resistance of the p contact layer and the contact resistance with the electrode must be reduced.

SUMMARY OF THE INVENTION

0010In view of the foregoing, an object of the present invention is to reduce the resistance of the p contact layer and the contact resistance with the electrode in a Group III nitride semiconductor light-emitting device.

0011In a first aspect of the present invention, there is provided a method for producing a Group III nitride semiconductor light-emitting device having a p contact layer, wherein a process for forming the p contact layer comprising:

0012a first process of forming a first p contact layer of GaN doped with Mg, by MOCVD using ammonia as a nitrogen source, hydrogen as a carrier gas;

0013a second process of lowering the temperature to a growth temperature of a second p contact layer being formed in the subsequent process, decreasing or stopping the supply of ammonia, and switching the carrier gas from hydrogen to nitrogen; and

0014a third process of forming a second p contact layer of InGaN doped with Mg on the first p contact layer by MOCVD using nitrogen as a carrier gas while maintaining the temperature of the second process.

0015Preferably, the growth temperature of the first p contact layer is 1000° C. or higher, and the growth temperature of the second p contact layer is 700° C. to 900° C. It is because when the growth temperature falls within this range, a p contact layer with good crystallinity can be formed. More preferably, the growth temperature of the first p contact layer is 1000° C. to 1050° C., and the growth temperature of the second p contact layer is 700° C. to 800° C. Further preferably, the growth temperature of the first p contact layer is 1000° C. to 1020° C., and the growth temperature of the second p contact layer is 700° C. to 750° C.

0016Preferably, the first p contact layer has a Mg concentration of 1×10¹⁹/cm³to 7×10¹⁹/cm³, and the second p contact layer has a Mg concentration of 2×10¹⁹/cm³to 2×10²⁰/cm³. When the Mg concentration falls within this range, a p contact layer can be formed with reduced resistance as well as with good crystallinity. The Mg concentration of the first p contact layer may be same as or different from that of the second p contact layer. More preferably, the first p contact layer has a Mg concentration of 4×10¹⁹/cm³to 7×10¹⁹/cm³, and the second p contact layer has a Mg concentration of 5×10¹⁹/cm³to 1×10²⁰/cm³.

0017Preferably, the first p contact layer has a thickness of 10 nm to 100 nm, and the second p contact layer has a thickness of 1 nm to 10 nm. When the thickness falls within this range, the resistance of the p contact layer as a whole can be reduced. More preferably, the first p contact layer has a thickness of 30 nm to 70 nm, and the second p contact layer has a thickness of 1 nm to 5 nm.

0018Preferably, the second p contact layer has an In composition ratio of 10 mol % to 20 mol % to number of mole of total Group III atoms. When the In composition ratio falls within this range, the contact resistance with an electrode, for example, an ITO transparent electrode being formed on the second p contact layer or a reflecting electrode formed of a high-reflectivity metal such as Ag, can be sufficiently reduced. More preferably, the In composition ratio is 15 mol % to 20 mol % to number of mole of total Group III atoms.

0019In the second process, a temperature lowering rate when lowering the temperature to the growth temperature of the second p contact layer is preferably 1° C./sec to 3° C./sec. When the temperature lowering rate falls within this range, nitrogen release from GaN crystal forming the first p contact layer <b>151</b> can be suppressed, thereby degradation of crystallinity can be prevented.

0020The supply of ammonia may be decreased or stopped after the temperature was lowered to the growth temperature of the second p contact layer or while lowering the temperature. Similarly, the carrier gas may be switched from hydrogen to nitrogen after the temperature was lowered to the growth temperature of the second p contact layer or while lowering the temperature. Switching the carrier gas while lowering the temperature is preferable because nitrogen release from the crystal can be further suppressed, and degradation of crystallinity can be prevented.

0021Before starting the third process after the second process, the temperature and the atmosphere are preferably maintained for one to ten minutes. Thereby, Mg can be further activated in the first p contact layer. More preferably, the temperature and the atmosphere are maintained for two to five minutes.

0022A second aspect of the present invention is drawn to a specific embodiment of the production method according to the first aspect, wherein the growth temperature of the first p contact layer is 1000° C. or higher, and the growth temperature of the second p contact layer is 700° C. to 900° C.

0023A third aspect of the present invention is drawn to a specific embodiment of the production method according to the first or second aspect, wherein the second p contact layer has an In composition ratio of 10 mol % to 20 mol % to number of mole of total Group III atoms.

0024According to the present invention, resistance reduction due to p-type conduction of the p contact layer and reduction of the contact resistance between the electrode and the p contact layer can be both achieved. As a result, the resistance of the Group III nitride semiconductor light-emitting device can be reduced. P-type activation of the first p contact layer <b>151</b> can be done during the successive processes for forming the second p contact layer, thereby the production process is simplified.

BRIEF DESCRIPTION OF THE DRAWINGS

0025Various other objects, features, and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood with reference to the following detailed description of the preferred embodiments when considered in connection with the accompanying drawings, in which:

0026<figref idref="DRAWINGS">FIG. 1</figref> shows the configuration of a Group III nitride semiconductor light-emitting device according to Embodiment 1; and

0027<figref idref="DRAWINGS">FIGS. 2A to 2C</figref> are sketches showing processes for producing the Group III nitride semiconductor light-emitting device according to Embodiment 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

0028Specific embodiments of the present invention will next be described with reference to the drawings. However, the present invention is not limited to the embodiments.

Embodiment 1

0029<figref idref="DRAWINGS">FIG. 1</figref> shows the configuration of a Group III nitride semiconductor light-emitting device according to Embodiment 1. The Group III nitride semiconductor light-emitting device according to Embodiment 1 includes a sapphire substrate <b>10</b>; and an n contact layer <b>11</b>, an n cladding layer <b>12</b>, a light-emitting layer <b>13</b>, a p cladding layer <b>14</b>, and a p contact layer <b>15</b>, each of the layers being formed of Group III nitride semiconductor, are sequentially deposited on the sapphire substrate <b>10</b> via an AlN buffer layer (not illustrated). Moreover, a trench having a depth extending from the top surface of the p-type contact layer <b>15</b> to the n contact layer <b>11</b>, and an n-electrode <b>16</b> is formed on the surface of the n contact layer <b>11</b> exposed at the bottom of the trench. An ITO transparent electrode <b>17</b> is formed on almost the entire top surface of the p contact layer <b>15</b>, and a p electrode <b>18</b> is formed on the transparent electrode <b>17</b>.

0030A concave and convex configuration such as a dot pattern or a stripe pattern may be formed on the surface at the n contact layer <b>11</b> side of the sapphire substrate <b>10</b> to improve light extraction performance. The sapphire substrate <b>10</b> may be replaced with a growth substrate formed from, for example, SiC, Si, ZnO, spinel, or GaN.

0031The n contact layer <b>11</b> is formed of n-GaN with a Si concentration of 1×10¹⁸/cm³or more. To reduce the contact resistance with the n-electrode <b>16</b>, the n contact layer <b>11</b> may be formed of multiple layers with different Si concentrations.

0032The n cladding layer <b>12</b> has a superlattice structure formed of fifteen layer units, each including an undoped InGaN layer (thickness: 4 nm), an undoped AlGaN layer (thickness: 0.8 nm), and a Si-doped n-GaN layer (thickness: 1.6 nm), which are deposited in this order. The initial layer of the n cladding layer <b>12</b>, that is, the layer being in contact with the n contact layer <b>11</b>, is an InGaN layer. The final layer of the n cladding layer <b>12</b>, that is, the layer being in contact with the light-emitting layer <b>13</b> is an n-GaN layer. The overall thickness of the n cladding layer <b>12</b> is 96 nm.

0033An ESD layer for improving electrostatic breakdown voltage of the device may be formed between the n contact layer <b>11</b> and the n cladding layer <b>12</b>. For example, the ESD layer has a three-layer structure including a first ESD layer, a second ESD layer, and a third ESD layer, the layers being sequentially deposited on the n contact layer <b>11</b>. The first ESD layer has pits (pit density: 1×10⁸/cm²or less) on the surface at the light-emitting layer <b>13</b> side thereof. The first ESD layer is formed of GaN having a thickness of 200 nm to 1000 nm and a Si concentration of 1×10¹⁶to 5×10¹⁷/cm³. The second ESD layer has pits (pit density: 2×10⁸/cm²or more) on the surface at the light-emitting layer <b>13</b> side thereof. The second ESD layer is formed of GaN having a thickness of 50 nm to 200 nm and a carrier concentration of 5×10¹⁷/cm³or less. The third ESD layer is formed of GaN, and has a characteristic value, as defined by the product of Si concentration (/cm³) and thickness (nm), of 0.9×10²⁰to 3.6×10²⁰(nm/cm³). Such a structure of the ESD layer can improve electrostatic breakdown voltage, emission performance, and reliability, and reduce the current leakage.

0034The light-emitting layer <b>13</b> has a MQW structure in which an undoped InGaN well layer and an undoped AlGaN barrier layer are alternately deposited in a repeated manner. A capping layer formed of AlGaN having an Al composition ratio below that of the barrier layer may be formed between the well layer and the barrier layer at the same growth temperature as employed for the well layer. When such a capping layer is provided, emission performance can be improved, since the capping layer prevents release of In from the well layer during heating for formation of the barrier layer. A layer formed of undoped GaN and undoped AlGaN may be formed between the light-emitting layer <b>13</b> and the p cladding layer <b>14</b> to prevent the diffusion of Mg from the p cladding layer <b>14</b> to the light-emitting layer <b>13</b>.

0035The p cladding layer <b>14</b> has a structure including seven layer units, each including a p-InGaN layer (thickness: 1.7 nm) and a p-AlGaN layer (thickness: 3.0 nm) which are sequentially deposited. The initial layer of the p cladding layer <b>14</b>, which is in contact with the light-emitting layer <b>13</b>, is the p-InGaN layer, and the final layer of the p cladding layer <b>14</b>, which is in contact with the p contact layer <b>15</b>, is the p-AlGaN layer. The overall thickness of the p cladding layer <b>14</b> is 32.9 nm. Mg is employed as a p-type impurity.

0036The p contact layer <b>15</b> has a structure including a first p contact layer <b>151</b> and a second p contact layer <b>152</b> which are sequentially deposited on the p cladding layer <b>14</b>. The first p contact layer <b>151</b> is formed of p-GaN, and the second p contact layer <b>152</b> is formed of p-InGaN. When the thickness and the Mg concentration of the first p contact layer <b>151</b> and the second p contact layer <b>152</b> fall within the following range, the overall resistance of the p contact layer <b>15</b> can be sufficiently reduced. The first p contact layer <b>151</b> has a thickness of 6 nm to 7 nm and a Mg concentration of 4×10¹⁹/cm³to 7×10¹⁹/cm³. The second p contact layer <b>152</b> has a thickness of 2 nm to 4 nm and a Mg concentration of 5×10¹⁹/cm³to 1×10²⁰/cm³. The second p contact layer <b>152</b> has an In composition ratio of 0.1 to 0.2. When the In composition ratio falls within this range, the second p contact layer <b>152</b> can be formed without degrading the crystallinity, and the contact resistance with an ITO transparent electrode <b>17</b> above and on contact with the second p contact layer <b>152</b> can also be reduced.

0037Next will be described processes for producing the Group III nitride semiconductor light-emitting device according to Embodiment 1 with reference to <figref idref="DRAWINGS">FIGS. 2A to 2C</figref>.

0038Firstly, on a sapphire substrate <b>10</b>, by MOCVD, a buffer layer (not illustrated), an n contact layer <b>11</b>, an n cladding layer <b>12</b>, a light-emitting layer <b>13</b>, and p cladding layer <b>14</b> are sequentially deposited (<figref idref="DRAWINGS">FIG. 2A</figref>). The gases employed are as follows: TMG (trimethylgallium) as a Ga source; TMI (trimethylindium) as an In source; TMA (trimethylaluminum) as an Al source; ammonia as a nitrogen source; and hydrogen or nitrogen as a carrier gas. Silane is employed as an n-type dopant gas.

0039Subsequently, on the p cladding layer <b>14</b>, by MOCVD, a first p contact layer <b>151</b> is formed of GaN doped with Mg (<figref idref="DRAWINGS">FIG. 2B</figref>). The gases employed are as follows: hydrogen as a carrier gas; TMG as a Ga source, and ammonia as a nitrogen source. Cp₂Mg (biscyclopentadienylmagnesium) is employed as a p-type dopant gas. The first p contact layer <b>151</b> has a thickness of 6 nm to 7 nm and a Mg concentration of 4×10¹⁹/cm³to 7×10¹⁹/cm³. The growth temperature is 1000° C. or higher. At this growth temperature, the first p contact layer <b>151</b> with good crystallinity can be formed. More preferably, the growth temperature is 1000° C. to 1020° C. The pressure is an atmospheric pressure. When the pressure falls within this range, the first p contact layer <b>151</b> with good crystallinity can be formed.

0040Next, after lowering the temperature to 700° which is the growth temperature of a second p contact layer <b>152</b> being formed in the subsequent process, the supply of ammonia is stopped, and the carrier gas is switched from hydrogen to nitrogen. The supply of TMG may not be stopped, or may be stopped. The pressure is the same as that when forming the first p contact layer <b>151</b>. The temperature of 700° C. and the nitrogen atmosphere are maintained for two to five minutes. Although such a holding time is not essential, it is preferable to set a holding time in order to sufficiently activate Mg. Thereby, Mg is activated in the first p contact layer <b>151</b>, and the first p contact layer <b>151</b> has a p-type conduction. Moreover, the p cladding layer <b>14</b> also has a p type conduction at the same time with the first p contact layer <b>151</b>.

0041The supply of ammonia may not be stopped but may be decreased to a certain amount. In this case, the supply amount of ammonia is preferably decreased to 10% or less. Otherwise, Mg is not sufficiently activated. Most preferably, the supply of ammonia is stopped. The supply of ammonia may be stopped or decreased while lowering the temperature or after the temperature was lowered. Similarly, the carrier gas may be switched while lowering the temperature or after the temperature was lowered. Switching the carrier gas while lowering the temperature is preferable because nitrogen release from the crystal can be further suppressed, and degradation of crystallinity can be prevented. A temperature lowering rate when lowering the temperature to 700° C. is preferably 1° C./sec to 3° C./sec. When the temperature lowering rate falls within this range, evaporation of nitrogen from GaN crystal forming the first p contact layer <b>151</b> can be effectively suppressed.

0042Subsequently, the temperature is maintained at 700° C., which is the temperature of the previous process, nitrogen is employed as a carrier gas, ammonia, TMG, TMI are supplied as a raw material gas, and Cp₂Mg is supplied as a p-type dopant gas. Under these conditions, by MOCVD, a second p contact layer <b>152</b> of InGaN doped with Mg is formed on the first p contact layer <b>151</b> (<figref idref="DRAWINGS">FIG. 2C</figref>). The second p contact layer <b>152</b> has a thickness of 2 nm to 4 nm and a Mg concentration of 5×10¹⁹/cm³to 1×10²⁰/cm³. The second p contact layer <b>152</b> has an In compositional ratio of 10 mol % to 20 mol % to total number of mole of Ga and In. That is 10≦100x≦20 in In_xGa_1-xN. The pressure when forming the second p contact layer <b>152</b> is atmospheric pressure Pa. When the pressure falls within this range, the first p contact layer <b>151</b> with good crystallinity can be formed.

0043The growth temperature of the second p contact layer <b>152</b> is 700° C. The temperature is not limited to this, but may be 700° C. to 900° C. When the growth temperature falls within this range, the first p contact layer <b>151</b> with good crystallinity can be formed.

0044Thereafter, a specific portion of the p contact layer <b>15</b> is subjected to dry etching, to thereby form a trench having a depth extending from the top surface of the p contact layer <b>15</b> to the n contact layer <b>11</b>. An ITO transparent electrode <b>17</b> is formed almost the entire top surface of the p contact layer <b>15</b>. Then, a p electrode <b>18</b> is formed on the transparent electrode <b>17</b>, and an n-electrode <b>16</b> is formed on the surface of the n contact layer <b>11</b> exposed at the bottom of the trench. Thus, the Group III nitride semiconductor light-emitting device shown in <figref idref="DRAWINGS">FIG. 1</figref> is produced.

0045At this time, the transparent electrode <b>17</b> comes in contact with the second p contact layer <b>152</b> of the p contact layer <b>15</b>. The second p contact layer <b>152</b> is formed of p-InGaN having an In composition ratio of 10 mol % to 20 mol % to number of mole of total Group III atoms. That is 10≦100x≦20 in In_xGa_1-xN. The p-InGaN has a smaller work function than p-GaN which forms the first p contact layer <b>151</b>. Therefore, a difference in work function between the second p contact layer <b>152</b> and the ITO transparent electrode <b>17</b> is smaller than in case of p-GaN. As a result, the contact resistance between the second p contact layer <b>152</b> and the transparent electrode <b>17</b> can be reduced.

0046The second p contact layer <b>152</b> is not subjected to thermal treatment to activate Mg and produce a p-type conduction. However, since the second p contact layer <b>152</b> is thin enough, the resistance of the p contact layer <b>15</b> can be sufficiently reduced as a whole.

0047According to the aforementioned method for growing the p contact layer <b>15</b>, the resistance of the first p contact layer <b>151</b> can be reduced, and the contact resistance can be reduced because the second p contact layer <b>152</b> being in contact with the transparent electrode <b>17</b>, is formed of InGaN. Thus, the resistance of the Group III nitride semiconductor light-emitting device according to Embodiment 1 can be reduced.

0048Moreover, p-type activation of the first p contact layer <b>151</b> can be continuously done during the successive processes for forming the second p contact layer <b>152</b>. Thereby, p-type activation after the formation of the second p contact layer <b>152</b> can be omitted. Thus, the production process is simplified.

0049The Group III nitride semiconductor light-emitting device according to Embodiment 1 is of a face-up type. However, the present invention is not limited to this, and a Group III nitride semiconductor light-emitting device may have any structure, so long as an electrode is formed on a p contact layer. For example, the present invention can be applied to a flip-chip type device or a device having a vertical conductive structure obtained by using a conductive substrate or removing a substrate by a laser lift-off technique. Thus, as in Embodiment 1, the contact resistance with the electrode as well as the resistance of the p contact layer can be reduced.

0050Usually, after the formation of the p contact layer, a plurality of thermal treatments are carried out such as thermal treatment for baking an ITO transparent electrode, or alloy treatment (thermal treatment) for obtaining an ohmic contact with a p electrode and an n electrode. They do not affect the effect of the present invention, that is, the p-type conduction of the p contact layer <b>11</b> and the reduction of the contact resistance with the electrode.

0051The Group III nitride semiconductor light-emitting device produced by the method of the present invention can be employed in, for example, an illumination apparatus.